The invention relates generally to an eddy current type braking system reduction device for assisting a frictional brake of a large vehicle, and more particularly, to a system in which magnet support rings retain permanent magnets in annularly equally spaced positions within a brake drum.
As disclosed, for example, in Japanese Patent Application Laid-Open No. 86061/1991, an eddy current type braking system in which a magnet support ring retaining a number of magnets is switched between a braking position opposed to an inner surface of the brake drum and a non-braking position withdrawn outwardly from the brake drum. In addition, Japanese Patent Application Laid-Open No. 88867/1992, discloses an eddy current type braking system in which a magnet support ring is rotatably disposed within a brake drum, and the ring is rotated forwardly and reversely by a half-array pitch of the magnets to thereby switch between a braking position and a non-braking position. This unit is shorter in full length (dimension in an axial direction of the brake drum) than that of the former, and if the axial width of the magnets is enlarged, it is possible to increase a braking performance.
However, in a large size truck or a large size bus, a drive transmission system often fails to provide much space for mounting of an eddy current type reduction device. It is therefore preferred that the eddy current type reduction device be made as small as possible. Miniaturization of the eddy current device, however, creates problems in connection with magnet width or desired alignment between a brake drum and pole pieces retained on an annular frame. That is, since the axial center of the ferromagnetic pole pieces on the guide frame do not align with the axial center of the brake drum, an uneven heat distribution pattern is produced, and thermal deformation tends to occur in the vicinity of the open end of the brake drum.